Enhancive and inhibitory effects of copper complexation on triplet dissolved black carbon-sensitized photodegradation of organic micropollutants

Photochemistry of plateau lake-derived dissolved organic matter: Reactive species generation and effects on 17β-estradiol photodegradation

Naturally strong ultraviolet irradiation at high altitudes causes photobleaching of plateau lake DOM (P-DOM) and affects its photochemical activity. However, the photoreactivity of P-DOM has remained unclear under natural photobleaching condition. Here, six P-DOM samples isolated from plateau lakes in Yunnan Province, China as well as two reference DOM as comparisons were used to explore the photogeneration of reactive species (RS) and their effects on 17β-estradiol photodegradation. Compared with SRHA/SRFA, P-DOM has lower aromaticity, average molecular weight, and electron-donating capacity. The quantum yields of triplet state P-DOM (3P-DOM*), 1O2, and ∙OH produced in P-DOM solutions were greatly higher than those of reference DOM. The RS quantum yields had positive linear correlations with E2/E3 and SR, whereas were negatively linear correlated with SUVA25. Radical quenching experiments showed that 3P-DOM* was the prominent RS for 17β-estradiol photodegradation, and its contribution exceeded 70% for each of P-DOM. 3P-DOM*-mediated photodegradation was mainly attributed to the electron-transfer reactions with an average second-order rate constant of 4.62 × 109 M-1s-1, indicating the strong photoreactivity towards 17β-estradiol. These findings demonstrate that P-DOM is an efficient photosensitizer for RS production, among which 3P-DOM* may play an important role in enhanced photodegradation for organic micropollutants in plateau lake enriched with DOM.

How does adsorptive fractionation of dissolved black carbon on ferrihydrite affect its copper binding behaviors? A molecular-scale investigation

Adsorptive fractionation of dissolved black carbon (DBC) on minerals is proven to alter its molecular composition, which will inevitably affect the environment fate of heavy metals. However, the effects of molecular fractionation on the interaction between DBC and heavy metals remain unclear. Herein, we observed that the selective adsorption of ferrihydrite caused molecular changes of DBC from high molecular weight/unsaturation/aromaticity to low molecular weight/saturation/aliphatics. This process accompanied by a retention of carbohydrate and a reduction of oxygen-rich functional groups (e.g., polyphenols and carboxyl) and long carbon chain in DBC. The residual DBC in aqueous phase demonstrated a weaker binding affinity to copper compared to the original DBC. This decrease in binding affinity was primarily attributed to the adsorption of polycyclic condensed aromatic compounds of 200-250 Da, oxygen-rich polycyclic condensed aromatic compounds of 250-300 Da, oxygen-rich non-polycyclic aromatic compounds of 300-450 Da, and non-polycyclic aromatic compounds of 450-700 Da in DBC by ferrihydrite. Additionally, the retention of carbohydrates and aliphatic compounds of 300-450 Da also made a significant contribution. Notably, carboxylic groups rather than phenolic groups were the dominant oxygen-containing functional groups responsible for this affinity reduction. This study has significant implications for understanding of the biogeochemical processes of DBC at soil-water interface and surface water, especially its role in the transportation of heavy metals.

Mechanistic and Kinetic Consideration of the Photochemically Generated Oxidative Organic Radicals in Dissolved Black Carbon Solutions under Simulated Solar Irradiation

The photochemically generated oxidative organic radicals (POORs) in dissolved black carbon (DBC) was investigated and compared with that in dissolved organic matter (DOM). POORs generated in DBC solutions exhibited higher one-electron reduction potential values (1.38-1.56 V) than those in DOM solutions (1.22-1.38 V). We found that the photogeneration of POORs from DBC is enhanced with dissolved oxygen (DO) increasing, while the inhibition of POORs is observed in reference to DOM solution. The behavior of the one-electron reducing species (DBC•-/DOM•-) was employed to explain this phenomenon. The experimental results revealed that the DO concentration had a greater effect on DBC•- than on DOM•-. Low DO levels led to a substantial increase in the steady-state concentration of DBC•-, which quenched the POORs via back-electron reactions. Moreover, the contribution of POORs to the degradation of 19 emerging organic contaminants (EOCs) in sunlight-exposed DBC and DOM solutions was estimated. The findings indicate that POORs play an important role in the photodegradation of EOCs previously known to react with triplets, especially in DBC solutions. Compared to DOM solutions, POOR exhibits a lower but considerable contribution to EOC attenuation. This study enhances the understanding of pollutant fate in aquatic environments by highlighting the role of DBC in photochemical pollutant degradation and providing insights into pollutant transformation mechanisms involving POORs.

Inhibiting mechanisms of metal ion complexation on photogenerated reactive intermediates derived from dissolved black carbon

Dissolved black carbon (DBC), an important photosensitizer in surface waters, can influence the photodegradation of various organic micropollutants. In natural water systems, DBC often co-occurs with metal ions as DBC-metal ion complexes; however, the influence of metal ion complexation on the photochemical activity of DBC is still unclear. Herein, the effects of metal ion complexation were investigated using common metal ions (Mn²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Zn²⁺, Al³⁺, Ca²⁺, and Mg²⁺). Complexation constants (logK_M) derived from three-dimensional fluorescence spectra revealed that Mn²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Zn²⁺, and Al³⁺ quenched the fluorescence components of DBC via static quenching. The steady-state radical experiment suggested that in the complex systems of DBC with various metal ions, Mn²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Zn²⁺ and Al³⁺ inhibited the photogeneration of ³DBC* via dynamic quenching, which reduced the yields of ³DBC*-derived ¹O₂ and O₂·^-. Moreover, ³DBC* quenching by metal ions was associated with the complexation constant. A strong positive linear relationship existed between logK_M and the dynamic quenching rate constant of metal ions. These results indicate that the strong complexation ability of metal ions enabled ³DBC quenching, which highlights the photochemical activity of DBC in natural aquatic environments enriched with metal ions.

The photoactivity of complexation of DOM and copper in aquatic system: Implication on the photodegradation of TBBPA

The photoactivity of dissolved organic matter (DOM) has a great impact on the photodegradation of organic pollutants in natural waters. In this study, the photodegradation of TBBPA was investigated under simulated sunlight irradiation in the presence of copper ion (Cu²⁺), dissolved organic matter (DOM) and Cu-DOM complexation (Cu-DOM) to illustrate the effect of Cu²⁺ on photoactivity of DOM. The rate of photodegradation of TBBPA in the presence of Cu-DOM complex was 3.2 times higher than that in pure water. The effects of Cu²⁺, DOM and Cu-DOM on the photodegradation of TBBPA were highly pH dependent and hydroxyl radical(·OH) responded for the acceleration effect. Spectral and radical experiments indicated that Cu²⁺ had high affinity to fluorescence components of DOM, and acted as both the cation bridge and electron shuttle, resulting the aggregation of DOM and increasing of steady-state concentration of ·OH (·OH_ss). Simultaneously, Cu²⁺ also inhibited intramolecular energy transfer leading to the decrease of steady-state concentration singlet oxygen (¹O_2ss) and triplet of DOM (³DOM^⁎_ss). The interaction between Cu²⁺ and DOM followed the order of conjugated carbonyl CO, COO^- or CO stretching in phenolic groups and carbohydrate or alcoholic CO groups. With these results, a comprehensive investigation on the photodegradation of TBBPA in the presence of Cu-DOM was conducted, and the effect of Cu²⁺ on the photoactivity of DOM was illustrated. These findings helped to understanding the potential mechanism of interaction among metal cation, DOM and organic pollutants in sunlit surface water, especially for the DOM-induced photodegradation of organic pollutants.